Christy Wells - who said she was friendly with the Coopers at that time - said Nancy told her about the relationship with a man she met on a trip to Florida.
Wells said Nancy told her she planned to leave Brad and the man would help her get a green card to stay in the country.
Under cross-examination, prosecutors pointed out that the Coopers continued in their marriage and had two children.
Wells said she and her former husband contacted Cary police to give them the information, but she felt that police were slow to respond.
Brad Cooper is charged with murdering Nancy in 2008. Her body was found dumped beside a drainage pond in an unfinished Cary subdivision not far from her home. An autopsy showed she'd been strangled.
Much of the trial has centered around problems within the Cooper marriage. Brad has admitted to having one affair himself and prosecutors have alleged relationships with other women.
Police got tip from psychic
Later Thursday, Lochmere Golf Club greens keeper Wayne Cuttler testified about a search by Cary police at the golf course.
He said officers asked him about large pipes or culverts on the course that might be in sight of colored flags.
In its opening statement, the Cooper defense said the search was triggered by a tip from a psychic.
Cuttler said he asked police why they wanted to search at his club.
"He told me that the information was from a phone call," Cuttler recalled.
Cuttler said he asked the officer if it was a tip from a psychic.
"He said he'd rather not say," said Cuttler.
Witness says she saw Nancy Cooper
Thursday afternoon, Cary resident Rosemary Zednick testified that she saw Nancy Cooper jogging the morning of the day she was reported missing.
Zednick said she was walking her dog when Cooper ran past her just after 7 a.m.
Brad Cooper told detectives Nancy went out for a run early Saturday July 12, but prosecutors say Brad killed her the night before.
Zednick said she picked Nancy's picture out of several others and she was sure of what she saw.
Zednick said Cooper had her hair pulled back and she thought she was wearing an iPod, but never saw it.
Prosecutors rested their case Monday. They allege Brad killed Nancy because he was angry she planned to divorce him and move with their two daughters to Canada.
Cooper's lawyers say he is innocent and have characterized the investigation by the Cary Police Department as inept. They say detectives focused on Cooper from the beginning of their investigation and never looked at other suspects.